The quilt patch titled Subject: mm: prevent poison consumption when splitting THP has been removed from the -mm tree. Its filename was mm-prevent-poison-consumption-when-splitting-thp.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------ From: Qiuxu Zhuo qiuxu.zhuo@intel.com Subject: mm: prevent poison consumption when splitting THP Date: Sat, 11 Oct 2025 15:55:19 +0800

When performing memory error injection on a THP (Transparent Huge Page) mapped to userspace on an x86 server, the kernel panics with the following trace. The expected behavior is to terminate the affected process instead of panicking the kernel, as the x86 Machine Check code can recover from an in-userspace #MC.

mce: [Hardware Error]: CPU 0: Machine Check Exception: f Bank 3: bd80000000070134 mce: [Hardware Error]: RIP 10:<ffffffff8372f8bc> {memchr_inv+0x4c/0xf0} mce: [Hardware Error]: TSC afff7bbff88a ADDR 1d301b000 MISC 80 PPIN 1e741e77539027db mce: [Hardware Error]: PROCESSOR 0:d06d0 TIME 1758093249 SOCKET 0 APIC 0 microcode 80000320 mce: [Hardware Error]: Run the above through 'mcelog --ascii' mce: [Hardware Error]: Machine check: Data load in unrecoverable area of kernel Kernel panic - not syncing: Fatal local machine check

The root cause of this panic is that handling a memory failure triggered by an in-userspace #MC necessitates splitting the THP. The splitting process employs a mechanism, implemented in try_to_map_unused_to_zeropage(), which reads the pages in the THP to identify zero-filled pages. However, reading the pages in the THP results in a second in-kernel #MC, occurring before the initial memory_failure() completes, ultimately leading to a kernel panic. See the kernel panic call trace on the two #MCs.

First Machine Check occurs // [1] memory_failure() // [2] try_to_split_thp_page() split_huge_page() split_huge_page_to_list_to_order() __folio_split() // [3] remap_page() remove_migration_ptes() remove_migration_pte() try_to_map_unused_to_zeropage() // [4] memchr_inv() // [5] Second Machine Check occurs // [6] Kernel panic

[1] Triggered by accessing a hardware-poisoned THP in userspace, which is typically recoverable by terminating the affected process.

[2] Call folio_set_has_hwpoisoned() before try_to_split_thp_page().

[3] Pass the RMP_USE_SHARED_ZEROPAGE remap flag to remap_page().

[4] Try to map the unused THP to zeropage.

[5] Re-access pages in the hw-poisoned THP in the kernel.

[6] Triggered in-kernel, leading to a panic kernel.

In Step[2], memory_failure() sets the poisoned flag on the page in the THP by TestSetPageHWPoison() before calling try_to_split_thp_page().

As suggested by David Hildenbrand, fix this panic by not accessing to the poisoned page in the THP during zeropage identification, while continuing to scan unaffected pages in the THP for possible zeropage mapping. This prevents a second in-kernel #MC that would cause kernel panic in Step[4].

Thanks to Andrew Zaborowski for his initial work on fixing this issue.

Link: https://lkml.kernel.org/r/20251015064926.1887643-1-qiuxu.zhuo@intel.com Link: https://lkml.kernel.org/r/20251011075520.320862-1-qiuxu.zhuo@intel.com Fixes: b1f202060afe ("mm: remap unused subpages to shared zeropage when splitting isolated thp") Signed-off-by: Qiuxu Zhuo qiuxu.zhuo@intel.com Reported-by: Farrah Chen farrah.chen@intel.com Suggested-by: David Hildenbrand david@redhat.com Acked-by: David Hildenbrand david@redhat.com Tested-by: Farrah Chen farrah.chen@intel.com Tested-by: Qiuxu Zhuo qiuxu.zhuo@intel.com Acked-by: Lance Yang lance.yang@linux.dev Reviewed-by: Wei Yang richard.weiyang@gmail.com Acked-by: Zi Yan ziy@nvidia.com Reviewed-by: Miaohe Lin linmiaohe@huawei.com Cc: Barry Song baohua@kernel.org Cc: Dev Jain dev.jain@arm.com Cc: Jiaqi Yan jiaqiyan@google.com Cc: Liam Howlett liam.howlett@oracle.com Cc: Lorenzo Stoakes lorenzo.stoakes@oracle.com Cc: "Luck, Tony" tony.luck@intel.com Cc: Mariano Pache npache@redhat.com Cc: Miaohe Lin linmiaohe@huawei.com Cc: Naoya Horiguchi nao.horiguchi@gmail.com Cc: Ryan Roberts ryan.roberts@arm.com Cc: stable@vger.kernel.org Signed-off-by: Andrew Morton akpm@linux-foundation.org ---

mm/huge_memory.c | 3 +++ mm/migrate.c | 3 ++- 2 files changed, 5 insertions(+), 1 deletion(-)

--- a/mm/huge_memory.c~mm-prevent-poison-consumption-when-splitting-thp +++ a/mm/huge_memory.c @@ -4109,6 +4109,9 @@ static bool thp_underused(struct folio * if (khugepaged_max_ptes_none == HPAGE_PMD_NR - 1) return false;

+ if (folio_contain_hwpoisoned_page(folio)) + return false; + for (i = 0; i < folio_nr_pages(folio); i++) { if (pages_identical(folio_page(folio, i), ZERO_PAGE(0))) { if (++num_zero_pages > khugepaged_max_ptes_none) --- a/mm/migrate.c~mm-prevent-poison-consumption-when-splitting-thp +++ a/mm/migrate.c @@ -301,8 +301,9 @@ static bool try_to_map_unused_to_zeropag struct page *page = folio_page(folio, idx); pte_t newpte;

- if (PageCompound(page)) + if (PageCompound(page) || PageHWPoison(page)) return false; + VM_BUG_ON_PAGE(!PageAnon(page), page); VM_BUG_ON_PAGE(!PageLocked(page), page); VM_BUG_ON_PAGE(pte_present(old_pte), page); _

Patches currently in -mm which might be from qiuxu.zhuo@intel.com are